1. Field of the Invention
The present invention relates in general to vacuum pumps and in particular to a new and useful rotary displacement pump which may be used to establish a high or ultra-high vacuum without contamination from lubricants or other unwanted gases produced by components of the pump.
2. Description of the Prior Art
Many applications require the production of high to ultrahigh vacuum, with a residual gas atmosphere free from carbon dioxide. To this end, mostly the prior art "dry" pumping systems are usually employed, such as ion getter pumps, sublimation pumps, cryogenic pumps, and turbo-molecular pumps. Further, to securely prevent a contamination of the container with hydrocarbons, it is desirable to avoid using oil-lubricated or oil-sealed forepumps during a pre-evacuation stage going from atmospheric pressure to a level at which these pumps start operating. That is why sorption pumps, for example, are employed which, however, are satisfactorily efficient only if a coolant such as liquid nitrogen is used. These pumps need regeneration so that their handling is too complicated to be suitable for manufacturing purposes where an automation of the pumping process is increasingly required.
What is needed is a pump which is instantly ready for operation in the same way as the mechanical pumps hitherto employed in industrial processes. The pump should be capable, without requiring a regeneration, of compressing to atmospheric pressure and discharging even large amounts of gas. On the other hand, the pump should not contain, in the pump chamber, any lubricants or sealing means which give off gas.
Attempts have already been made to use mechanical forepumps, such as piston valve pumps with dry running carbon sliders or Roots-type pumps, where hydrocarbons are avoided as sealing means in the pump chamber. As long as such pumps are provided with lubricated bearings, however, they are never completely secure against leakage of the lubricant, not even with the usual sealing sleeves or sliding packings between the bearings and the pump chamber. A completely satisfactory solution is obtained only with a truly gastight separation of the bearings from the pump chamber, for example, by means of resilient metallic or plastic bodies. Since such bodies may be allowed only to bend and must not be subject to torsion, the sole kind of motion resulting therefrom, for the displacer performing the pumping, is an oscillatory movement without rotation.
Even though the bearing and drive problem may be solved in this way, still further factors are to be taken into account in high-speed dry pumps:
In principle, sliding parts producing abrasion and heat in long-term operation should be avoided. From this requirement it follows that the gaps should be as long as possible, since their width cannot drop below a certain minimum given by the manufacturing tolerances and the unequal coefficient of expansion of the component parts. This is done so as to obtain, in spite of the absent lubricant and sealing means, low conduction values between the discharge and suction sides of the pump.
Should sliding parts not be avoidable for obtaining a high compression ratio, at least the contact pressure of the surfaces sliding on each other and their sliding speed should be minimized.
Another important requirement is to have a small dead space in the pump, since only then can high compression ratios and low ultimate pressures be obtained with few stages. Also, additional measures should be taken at the suction side of the pump to prevent a back flow of gases which have already been displaced, to the intake.
There have already been provided dry mechanical pumps in which the above described separation of the drive and the bearings from the pump chamber is effected by means of metallic or plastic spring bodies. Such is the design, for example, of a pump with a cylindrical pump chamber according to Luxembourgian Pat. No. 53323 or the U.S. Pat. No. 3,782,865 and of a pump with a spiral pump chamber according to Swiss Pat. No. 514,787. The first-named design does avoid sliding parts in the delivery space, but has the disadvantage of a large dead volume at the discharge side. This causes a permanent high pressure difference between the discharge side and the suction side leading to considerable leakage losses in the seal gaps and resulting in too low a compression ratio. To obtain the desired ultimate pressure, many such pump stages would have to be connected in series and the costs would rise correspondingly.
In the pump with a spiral pump chamber, the conditions are substantially better in this respect. Its application on a large scale, however, is hindered by the fact that such an embodiment is relatively complicated and expensive in manufacture, since relatively close manufacturing tolerances are to be observed in the absence of sealing means.